This invention relates to a wireless link changeover control method (handoff control method) carried out when a mobile station (a car telephone, a portable telephone, etc.) moves from one cell to another cell. More particularly, the invention relates to a CDMA (Code Division Multiple Access) soft handoff control method.
In a cellular mobile telecommunications system, control for changing over a wireless link when a mobile station (a car telephone, a portable telephone, etc.) moves from one cell to another cell is referred to as xe2x80x9chandoffxe2x80x9d. One method of such handoff control is soft handoff.
According to ordinary handoff (referred to as xe2x80x9chardxe2x80x9d handoff as opposed to xe2x80x9csoftxe2x80x9d handoff), the wireless link between the mobile station and a base station in a first cell is severed at the time of handoff, after which a wireless link is established between the mobile station and a base station in a second cell. By contrast, soft handoff establishes a wireless link between the mobile station and the base station in the second cell while the wireless link between the mobile station and the base station in the first cell is maintained. In other words, soft handoff can be thought of as a handoff technique characterized by the fact that wireless links are established with two (or more than two) base stations at the same time. This technique is often used in CDMA schemes (see the specification of International Application Laid-Open No. WO91/07020).
FIG. 11 is a diagram showing the configuration of a mobile telecommunications system, and FIG. 12 is a diagram useful in describing the soft handoff procedure. The system shown in FIG. 11 includes a mobile station (MS) 1, base stations (A, B) 2, 3 which communicate wirelessly with the mobile station in cells 4, 5, respectively, and a base station controller (BSC) 6. When the mobile station 1 in cell 4 moves and approaches the side of base station B while communicating with the base station A, the strength of radio waves received from the base station B gradually increases. When the mobile station 1 arrives at the region where the cells 4 and 5 intersect and the electric field strength of the radio waves received from the base station B surpasses a set level, the mobile station 1 notifies the base station controller 6 of this fact via the base station 2. As a result, the base station controller 6 establishes a wireless link between the mobile station 1 and the base station B without severing the wireless link between the mobile station 1 and the base station A, and the mobile station 1 therefore communicates with the plurality of base stations A, B simultaneously. When the reception field strength between the mobile station 1 and the base station A falls below the set level and remains there for a predetermined period of time, the wireless link between them is severed and the mobile station 1 communicates with the base station B from this point onward.
When the strength of a pilot signal from the base station B exceeds a set level T-ADD in a CDMA scheme, the mobile station 1 sends a pilot strength measurement message to the base station controller BSC via the base station A (1a, 2a in FIG. 12). Upon receiving this message, the base station controller BSC judges whether handoff between the mobile station 1 and the base station B should be executed. If the base station controller BSC decides to execute handoff, then it instructs the base station B of a communication channel CH, etc., allocated to the mobile station 1. In response, the base station B starts the transmission of a channel designating message on a channel Forward T-CH (2b).
Next, the base station controller BSC transmits a handoff direction message to the mobile station 1 via the base stations A and B (2c). Upon receiving the handoff direction message, the mobile station 1 acquires the communication channel CH, on which transmission is taking place, from the base station B and thenceforth is capable of communicating with the base station B over this channel and sends a handoff completion message to the base station controller BSC via the base station A and B (1b, 2d).
The mobile station 1 thenceforth communicates with the base station A, B simultaneously. If the strength of the pilot signal from the base station A falls below the set level and remains there for a set period of time, the mobile station 1 transmits the pilot strength measurement message to the base station controller BSC via the base station A. In response to this notification, the base station controller BSC decides to terminate communication between the mobile station 1 and the base station A and transmits the handoff direction message to the mobile station 1 via the base stations A, B (1c, 2e).
Upon receiving the handoff direction message, the mobile station 1 transmits the handoff completion message to the base station controller BSC via the base stations A, B and severs the wireless link with the base station A (1d).
Upon receiving the handoff completion message via the base stations A, B, the base station controller BSC instructs the base station A to stop using Forward T-CH. The base station A responds by stopping transmission/reception on Forward T-CH (2f). From this point onward the mobile station 1 communicates with the base station B.
Soft handoff has the following two characterizing features:
(1) Hitless handoff is possible.
Unlike hard handoff, wireless links can be changed over without conversation being interrupted at the time of handoff.
(2) Inter-cell diversity is possible.
Wireless link quality within a cell usually is poor near the perimeter of the cell. However, enabling communication with a plurality of base stations simultaneously by soft handoff makes possible inter-cell diversity (namely communication in which the base station that is capable of communicating at the lowest power is selected for communication). This reduces susceptibility to the effects of shadowing and fading.
Feature (1) mentioned above generally is noted as being the advantage of soft handoff. In actuality, however, feature (2) is more important because it increases the number of channels that can be accommodated by a base station (i.e., the channel capacity of the base station), as evident from the analysis set forth in the literature [A. J. Viterbi and A. M. Viterbi, xe2x80x9cOther-Cell Interference in Cellular Power-Controlled CDMAxe2x80x9d, IEEE Trans. on. Commun., Vol. 42, No. 2/3/4/(1994)]. Specifically, if soft handoff is not carried out in CDMA uplink (the wireless link from the mobile station to the base station), interference on other cells increases greatly and the number of wireless channels that can be used decreases significantly (i.e., subscriber capacity declines greatly). In order to assure CDMA uplink channel capacity, therefore, soft handoff is an essential technique. Since communication with soft handoff is performed upon selecting the base station that is capable of communicating at the lowest power, interference with other cells is reduced and the number of wireless channels (channel capacity) that can be used is increased.
FIGS. 13A, 13B are diagrams useful in describing the effect of raising uplink channel capacity by soft handoff, in which FIG. 13A shows a case where soft handoff is not carried out and FIG. 13B a case where soft handoff is carried out.
Consider the situation where soft handoff is not performed (FIG. 13A). If a great deal of shadowing is caused by a building BLD or some other structure between a mobile station MS1 and a base station BS1 in a cell CL1, the mobile station MS1 controls transmission power in such a manner that the incoming call level at the base station BS1 will take on a stipulated value. Transmission power of the mobile station MS1 rises as a result. When this takes place, interference (see the arrow indicated by the dashed line) with respect to a cell of interest (cell CL0) increases and a limitation is imposed upon the number of mobile stations accommodated by the cell.
Now consider the situation where soft handoff is carried out (FIG. 13B). Even if shadowing occurs between the mobile station MS1 and base station BS1 in cell CL1, it will suffice if the mobile station MS1 can communicate with the base station BS2 and, hence, the transmission power of the mobile station MS1 need not be very high. This reduces the interference on the cell of interest CL0 and no limitation is imposed upon the number of mobile stations that can be accommodated by the cell, unlike the case where soft handoff is not used.
If the principal purpose of soft handoff is to provide greater convenience in CDMA schemes, as in feature (1) mentioned above, there is some margin available in terms of selecting whether soft handoff is to be performed or not. In order to assure channel capacity in CDMA schemes, however, soft handoff is essential and there is no leeway in terms of selecting it. This means that soft handoff must be implemented as simply and as inexpensively as possible.
Control according to the prior art is simple. Specifically, the mobile station measures the strength of signal power from the base station and, when the received power is less than a certain threshold value, the mobile station judges that the distance between the mobile station and the base station has become too great and performs soft handoff accordingly. In effect, the mobile station when situated at the central portion of a cell at which it will not cause interference in other cells does not perform soft handoff; only when it is at the perimeter of the cell where it will interface with other cells does the mobile station carry out soft handoff. However, this conventional technique is not an optimum soft handoff allocation scheme which maximizes channel capacity. In particular, a problem which arises is that if the soft handoff rate is low, uplink channel capacity undergoes a marked decline. It should be noted that soft handoff rate h is given by the following equation:
h=(NCHxe2x88x92M)/Mxe2x80x83xe2x80x83(1)
where NCH represents the number of channels physically possessed by the base station and M represents the number of mobile stations which a base station can accommodate. In other words, the soft handoff rate is the ratio of the number (NCHxe2x88x92M) of wireless channels allocated for soft handoff use to the number M of mobile stations.
Accordingly, an object of the present invention is to provide an optimum soft handoff control method that maximizes channel capacity and, more particularly, a soft handoff control method performed so as to maximize channel capacity in dependence upon the soft handoff rate of a base station.
Another object of the present invention is to provide a soft handoff control method performed in such a manner that a variation in channel capacity is reduced even if the soft handoff rate of a base station changes.
In accordance with the present invention, the foregoing objects are attained by providing a soft handoff control method in a CDMA mobile telecommunications system in which, when a mobile station is changed over from a wireless link with a certain base station to a wireless link with another base station, a changeover to a prescribed wireless link is performed after wireless links are established with a plurality of base stations simultaneously, comprising the steps of (1) entering a soft handoff rate of a base station and (2) controlling, on the basis of the soft handoff rate, a maximum number of soft handoff branches allocated to a mobile station residing in a cell that corresponds to the above-mentioned base station.
For example, it is so arranged that soft handoff control is carried out at the perimeter of a cell and reception field strength conforming to a boundary between the cell perimeter and the cell center is set. When the reception field strength of radio waves from another base station exceed the set level, soft handoff control is executed in accordance with the maximum number of soft handoff branches. This arrangement makes it possible to perform soft handoff control so as to maximize the channel capacity of the base station.
In accordance with the present invention, the foregoing objects are attained by providing a soft handoff control method in a CDMA mobile telecommunications system in which, when a mobile station is changed over from a wireless link with a certain base station to a wireless link with another base station, a changeover to a prescribed wireless link is performed after wireless links are established with a plurality of base stations simultaneously, comprising a step of controlling, on the basis of the position of the mobile station in a cell conforming to base station, a number of soft handoff branches allocated to the mobile station.
For example, the number of soft handoff branches allocated to the mobile station is decided based upon the position of the mobile station in a cell so as to obtain the maximum channel capacity and reduce a variation in the channel capacity with respect to the soft handoff rate. Soft handoff control is then performed in accordance with this number of soft handoff branches. More specifically, in order that the maximum channel capacity will be obtained and a variation in the channel capacity with respect to the soft handoff rate reduced, a cell is divided into a cell perimeter area, one or more cell intermediate areas and a cell center area, the number of soft handoff branches in each area is decided in advance and the number of soft handoff branches allocated to the mobile station is controlled based upon the area in which the mobile station is situated.